Modern implantable cardiac pacing devices are designed for efficient dual or multiple chamber pacing as well as detection and treatment of dangerous cardiac arrhythmias. A dual chamber pacing device provides the capability of synchronous pacing, whereby the ventricle is paced in synchrony with the just preceding atrial beat (intrinsic or paced), thereby approximating the normal healthy coordination between the atrium and the ventricle and thus optimizing cardiac output. However, if the atrium is seized with an arrhythmia, such synchronous pacing cannot be resumed until after the arrhythmia abates or is somehow reverted. In the case of atrial tachycardia (AT), a malignant arrhythmia, pacemakers can respond with normally effective atrial pacing schemes to arrest or stop the AT, after which the pacemaker can resume synchronous pacing when the cardiac condition is stabilized. The problem is that the atrium remains vulnerable to a recurring episode of tachycardia following AT treatment, particularly if the two chambers are not operating in an efficient synchronous manner. What is important, then, and a main feature of this invention, is enabling the pacemaker or other implantable device (ipg) to resume such synchronous pacing as quickly as possible after the conclusion of the AT response.
The advantage of synchronous pacing is well established. For a patient at rest, AV synchrony improves cardiac output by about 20–25%; the improvement decreases with exercise. The intrinsic atrial rate, however, is not always reliable for control of ventricular pacing. Pacemaker patients are, by definition, in a class that is subject to cardiac abnormalities. In particular, AT is a concern, and the pacemaker or ipg must be able to detect AT and provide an appropriate response to terminate the AT. As used here, AT means an abnormally high atrial rate, e.g., a rate of intrinsic atrial beats above 120–150 bpm. The definition of AT may be programmed into the device by a physician, and may be set at a value within a range of, for example, 100–180 bpm. Most pacing devices have a programmable upper rate limit above which the ventricle will not be paced, and this limit may serve to define tachy senses. In a typical arrangement, when such high rate tachy senses occur consecutively or in predominance over a given time period, AT is recognized and an anti-tachy pacing (ATP) mode automatically takes over to stop the AT.
There are many different ATP schemes illustrated in the patent art. Table 1 below lists representative patent and literature references that show different forms of ATP.
TABLE 1Patent/Doc. No.Inventor(s)Issue/Pub Date4,280,502Baker, Jr. et al.Jul. 28, 19814,390,021Spurrell et al.Jun. 28, 19834,398,536Nappholz et al.Aug. 16, 19834,406,287Nappholz et al.Sep. 27, 19834,408,606Spurrell et al.Oct. 11, 19834,467,810VollmannAug. 28, 19844,574,437Segerstad et al.Mar. 11, 19864,577,633Berkovitz et al.Mar. 25, 19864,587,970Holly et al.May 13, 19864,593,695WittkampfJun. 10, 19864,491,471MehraJul. 17, 1990
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
A common form of ATP is to deliver a train of pulses to the atrium immediately after AT is declared. In delivering an ATP train, or burst, different sequences of atrial pulses can be used, with the timing being adjusted for maximum interruption of the tachycardia. Atrial tachycardia is thought to be characterized by a re-entry feedback loop in the atrium, whereby each atrial conduction induces another beat before the natural pacemaker triggers the next normal beat. The timing of the feedback conduction can be variable, such that it is difficult to determine the best time to deliver a reverting pulse that can interrupt the arrhythmia. The basic idea of the train is to deliver a series of pulses before the next tachy occurrence, to enhance the possibility of interruption. It is thought that each successive pulse of the sequence enlarges the area of the atrium that is conditioned to respond to a pacing pulse, such that by the end of the train the entire atrium is in condition to be captured by an atrial pulse (AP). The sequence, whether called a train or burst or whatever, can be programmed for each patient, and may even vary based on history. As used herein, ATP refers to any sequence of atrial pulses delivered for the purpose of interrupting atrial tachycardia or like arrhythmias. However, the various ATP trains have in common the features that atrial sensing is abolished while the ATP pulses are being delivered, and once the train is started the order and timing of the ATP pulses is set, i.e., the train is irrevocable.
As stated above, it is important to continue with synchronous pacing as soon as possible after an AT episode. In fact, establishing synchronous pacing quickly after an abnormal episode is crucial, due to the possibility of mediating or permitting re-establishment of the tachycardia. By way of example, following a premature atrial contraction (PAC) it is known to deliver an atrial sync pulse (ASP) that is timed to enable the pacer to re-establish snychrony with the next delivered ventricular pulse. This is done in order to prevent pacemaker mediated tachycardia. Likewise, following an AT episode, there is a danger of a new AT episode if the next ventricular pace pulse is not synchronized to the last atrial pulse of the ATP. An asynchronous ventricular pulse could put stress on the atrium, which increases the vulnerability to AT or AF. Or, such an asynchronous pulse could lead to retrograde conduction back to the atrium which could induce AT or AF. Moreover, these two mechanisms could stimulate the localized source of polarization that gave rise to the AT or AF in the first place. Thus, while the ATP has presumably stopped the feedback mechanism in the atrium that was admitting the AT, the atrium remains vulnerable following AT and a following asynchronous ventricular pulse can cause either electrical or mechanical interference capable of destabilizing the atrial tissue. This presents a serious problem that has not been addressed by the pacemaker art.